I can see (from other questions here) how to add title, author, keywords etc. However, I'm creating transcriptions of historical documents and I'd like to add some additional metadata, such as the record office holding the document and the document accession number.

The idea is that (in the future) when I want to make them available on the web I can write an app that can read the PDF to extract the metadata (via something like PDFBox or iText) to include it in a HTTP POST.